1. Field of the Invention
This invention relates to methods and apparatus for processing and cleaning a substrate, and more specifically to methods and apparatus for cleaning semiconductor substrates after polishing of copper films.
2. Background Information
In the manufacture of advanced semiconductor devices, copper (Cu) is beginning to replace aluminum (Al) as the material for metallization. Cu has become desirable due to its lower resistivity and significantly improved electromigration lifetime, when compared to Al.
One process for Cu metallization uses a dual damascene approach. As illustrated in FIG. 1a, a dielectric layer 110 is deposited above a substrate 100. Dielectric layer 110 may be made up of materials such as silicon dioxide. Vias and/or trenches 120 are then formed in the dielectric layer 110, as illustrated in FIG. 1b. Vias/trenches 120 may be formed, for example, using dry etching techniques. Next, a thin layer of barrier material (barrier layer) 130, for example, tantalum (Ta), titanium (Ti), or titanium nitride (TiN) is deposited as illustrated in FIG. 1c. After barrier layer 130 is deposited the vias/trenches 120 are filled with copper (Cu) layer 140, as illustrated in FIG. 1d. Cu layer 140 may be deposited using well known deposition techniques, for example, chemical vapor deposition (CVD), physical vapor deposition (PVD), or electroplating. In order to isolate the copper interconnects, as illustrated in FIG. 1e, the excess copper layer 140 and barrier layer 130 must be removed.
One method for removing the excess copper layer 140 and barrier layer 130 is polishing the surface of the substrate, for example, polishing using chemical mechanical polishing (CMP). In a CMP process, the semiconductor substrate is polished with a slurry containing abrasive particles, such as alumina particles, and an oxidant, such as hydrogen peroxide. In the CMP process, contaminants are introduced which include particles and/or metal contamination on the copper layer 150, dielectric surface 160, and in the dielectric subsurface 165.
Regardless of how the CMP process is performed, the surface of semiconductor substrate must be cleaned of contaminants. If not removed, these contaminants may affect device performance characteristics and may cause device failure to occur at faster rates than usual. Cleaning the semiconductor substrate after chemical mechanical polishing of copper may be necessary to remove such contaminants from the copper layer and dielectric layers.
One method for cleaning the semiconductor substrate after polishing of the copper layer is brush scrubbing. Brush scrubbing, whether single-sided or double-sided brush scrubbing, is the industry standard for cleaning oxide and tungsten CMP applications. However, there are several problems associated with applying brush scrubbing to post copper CMP cleaning.
One such problem is brush loading. During the CMP process, the top surface of the copper layer may be oxidized and forms copper oxide, for example copper oxide (Cu2O or CuO) or copper hydroxide (Cu(OH)2). In basic or neutral pH cleaning environments, the copper oxide or copper hydroxide does not dissolve and may be transferred to the brushes, thus loading the brushes. The contaminated (or loaded) brushes may then transfer the copper oxide or copper hydroxide contaminants to subsequently processed substrates during cleaning.
For tungsten and other oxide applications, brush loading could be curtailed by adding a dilute ammonium hydroxide (NH4OH). In the presence of NH4OH, part of the copper oxide may form Cu(NH3)2+ complex and may be dissolved; however, due to the high pH environment, the dilute ammonium hydroxide has been found to be insufficient to prevent brush loading of copper oxide. Additionally, it has been found that scrubbing with dilute ammonium hydroxide also causes etching of the copper layer and may cause serious surface roughening.
Brush loading may also occur when alumina particles are used in the copper CMP process. In neutral or inorganic acid (e.g. HCl) cleaning environments, there is an electrostatic attraction between alumina particles and the silicon dioxide surface which makes it difficult to remove the alumina particles from the surface of the dielectric material. Because of the electrostatic attractive force, the alumina particles may also adhere to the brush and cause another brush loading problem with similar effects to those discussed above.
Yet another problem caused by the CMP process is that the surface and subsurface of the dielectric layer may become contaminated during polishing with metal from the copper layer and barrier layer as well as other contaminants from the slurry. During the CMP process, contaminants, especially metal contaminants, may penetrate into the dielectric layer up to approximately 100 angstroms (xc3x85) from the surface. Again, these contaminants may affect device performance characteristics and may cause device failure.
Thus, what is needed is a cleaning environment and methods for cleaning post copper CMP substrates that alleviate the problems of brush loading without affecting the quality of the copper and dielectric layers. Furthermore, what is needed is a cleaning environment and methods for cleaning post copper CMP substrates that have the capability of removing surface and subsurface contaminants from the copper and dielectric layers.
In one aspect of the invention, a cleaning solution for cleaning a semiconductor substrate is provided. The cleaning solution may be formed by mixing an amount of citric acid, an amount of ammonium fluoride, and an amount of hydrogen fluoride in deionized water. In one embodiment, the amount of citric acid is in a range from about 0.09% by weight to about 0.11% by weight, the amount of ammonium fluoride is in a range from about 0.4% by weight to about 0.6% by weight, and the amount of hydrogen fluoride is in a range from about 0.09% by weight to about 0.11% by weight, and the cleaning solution has a pH of about 4.
In another aspect of the invention, a method for cleaning a semiconductor substrate having a polished copper layer is provided. In this method, a concentrated cleaning solution is provided. The semiconductor substrate having the polished copper layer is placed in a scrubbing apparatus. In one embodiment, one unit volume of the concentrated cleaning solution is mixed with 20 unit volumes of deionized water proximate to the scrubbing apparatus to obtain a cleaning solution. The semiconductor substrate is scrubbed in the scrubbing apparatus in the presence of the cleaning solution.
Additional features and benefits of the present invention will become apparent from the detailed description, figures, and claims set forth below.